Suggest replacing braces for brackets on array-esque invalid block expr

Newcomers may write `{1, 2, 3}` for making arrays, and the current error
message is not informative enough to quickly convince them what is
needed to fix the error.
This PR implements a diagnostic for this case, and its output looks like
this:
```text
error: this code is interpreted as a block expression, not an array
 --> src/lib.rs:1:22
  |
1 |   const FOO: [u8; 3] = {
  |  ______________________^
2 | |     1, 2, 3
3 | | };
  | |_^
  |
  = note: to define an array, one would use square brackets instead of curly braces
help: try using [] instead of {}
  |
1 | const FOO: [u8; 3] = [
2 |     1, 2, 3
3 | ];
  |
```

Fix #87672

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